Infertile Sons May
Have Mom to Blame

Poor Mom. Already responsible for carrying genes that cause everything from hemophilia to colorblindness in her sons, she might be to blame when it comes to her sons' infertility, too.

Scientists have found that almost half the genes related to sperm production reside in the X chromosome, universally thought of as the female sex chromosome. It had been assumed that genes governing male fertility, if they were specialized at all, were from the Y, or male sex chromosome.

This could mean that the X chromosome could affect male infertility, an area that has never before been explored.

"The door is now wide open to see if there are links on the X chromosome to male infertility," says Dr. David Page, a biology professor at the Whitehead Institute for Biomedical Research in Cambridge, Mass., and lead author of a study published in this month's Nature Genetics.

"It's rare in science to stumble into a completely unvisited valley," he says. "The Y chromosome has an outsize role in sperm production, but now it looks like the X chromosome has a specialized role. We certainly didn't anticipate this."

To date, research on male infertility has focused primarily on abnormalities in the Y chromosome, but that only accounts for 5 percent to 10 percent of the causes of male infertility, Page says.

In an animal study with mice, researchers at the Whitehead Institute and Howard Hughes Medical Institute found 25 genes -- including 19 new ones -- in mouse sperm cells. Of those only three were linked to the Y chromosome, while 10 were linked to the X chromosome. The rest were linked to non-gender specific chromosomes. Later, the scientists were identified the same pattern of chromosome links in humans.

A lot more work needs to be done before any conclusions can be made about how much the X chromosome can affect male infertility, Page says.

"We don't yet know that mutations in the X chromosome cause male infertility, but these studies open the door to the possibility," he says.

"This is clearly an important finding because people have been focusing on the Y chromosome," says Dr. Margaret McGovern, associate professor of genetics at Mount Sinai School of Medicine in New York City. "Geneticists will start thinking differently about these patterns of genetics. Mothers' uncles and brothers are people who haven't been looked at before [in searching for causes for male infertility]. Now they will be encouraged."

The X-linked mode of inheritance is one of three modes of inheritance that doctors study, Page says. In this mode, a genetic defect on the X-chromosome may cause a disease like hemophilia or colorblindness, one of hundreds of X-linked diseases. The mother with a defective gene on one of her two X chromosomes is protected against the disease herself, because she has two X chromosomes and the normal X makes up for the defective one.

However, if her son inherits the defective X chromosome -- which he has a 50 percent chance of doing -- he is likely to get the disease because he doesn't have the balancing chromosome that a woman would. A daughter inheriting the defective X chromosome has another healthy X chromosome, like her mother, to protect her from the disease, but she can in turn pass the defective chromosome onto her own sons.

Any breakthroughs in treating male infertility are way down the road, but this study opens an exciting avenue of new research.

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